The scientific and technological advantages of nanostructured particles and materials have been attracting considerable attention. The small size of nanoparticles (generally used to indicate particles less than 100 nm in diameter), which can be responsible for different useful properties (electronic, optical, electrical, magnetic, chemical, and mechanical), makes them suitable for a wide variety of industrial applications.
Titanium dioxide (TiO2) nanoparticles are substantially transparent to visible light but can absorb and scatter ultraviolet light. Titanium dioxide has low toxicity and is non-irritating to the skin. TiO2 nanoparticles are especially advantageous when added to products in which transparency to visible light is important but exposure to the degrading and harmful effects of ultraviolet light is a problem. Such applications include, without limit, cosmetics, sunscreens, protective coatings, such as clear coatings for exterior wood and automobiles, and plastics.
Manufacture of nanoparticulate TiO2 has been reported throughout the literature. Sulfate-route or liquid phase precipitation routes typically involve nanoparticulate TiO2 particle growth steps, followed by aggregation, from calcinination or other high temperature treatment, with subsequent milling to reduce and/or control finished product particle size at the optimum required for performance. A high temperature plasma oxidation of titanium tetrachloride (TiCl4) process in which nanoparticulate TiO2 is manufactured directly under suitable reactor design and operating conditions has been described. As these processes require high energy costs, alternative means of manufacture would be desirable.
Titanium dioxide itself is known to be photoactive. Free radicals form on the surface of the titanium dioxide particle under the action of ultraviolet rays. While the photoactivity of titanium dioxide is beneficial for use of titanium dioxide in photo catalyzed reactions, in other uses the free radicals can lead to degradation reactions and yellowing which can be disadvantageous. Such other uses include, without limit, cosmetics, sunscreens and plastics, wood and automotive coatings, etc. Thus, there is a desire for techniques that can photo-passivate the titanium dioxide; that is, render the titanium dioxide more photostable.
Untreated titanium dioxide pigments and nanoparticles are known to be chemically reactive. Untreated titanium dioxide will form highly colored complexes with certain antioxidants, such as ascorbic acid and ascorbic acid 6-palmitate. These colored complexes limit the use of titanium dioxide nanoparticles in applications where white creams and lotions are desired, such as cosmetics and sunscreens. Effective methods for passivation of the chemical reactivity of titanium dioxide pigments and nanoparticles are therefore desired.
Treatments to passivate TiO2 will typically cause agglomeration of primary particles. Although steps can be taken to reduce this agglomeration, there is, typically, required a downstream grinding or milling step to reduce particle agglomeration to the range necessary for optimum performance. Treatment techniques that increase agglomeration can necessitate more intensive grinding or milling steps that add significantly to the energy costs of nanoparticulate TiO2 manufacture. Thus, there is a need for techniques that can make titanium dioxide nanoparticles nonreactive to such antioxidants and that have the required particle size without the need for long milling times that would add to the cost of manufacturing these nanoparticles.